Solvent based environmental barrier coatings for high temperature ceramic components

ABSTRACT

Environmental barrier coatings for high temperature ceramic components including a bond coat layer; an optional silica layer; and at least one transition layer including: from about 85% to about 100% by volume of the transition layer of a primary transition material including a rare earth disilicate, or a doped rare earth disilicate; and from 0% to about 15% by volume of the transition layer of a secondary material selected from Fe2O3, iron silicates, rare earth iron oxides, Al2O3, mullite, rare earth aluminates, rare earth aluminosilicates, TiO2, rare earth titanates, Ga2O3, rare earth gallates, NiO, nickel silicates, rare earth nickel oxides, Lnb metals, Lnb2O3, Lnb2Si2O7, Lnb2SiO5, borosilicate glass, alkaline earth silicates, alkaline earth rare earth oxides, alkaline earth rare earth silicates, and mixtures thereof; where the transition layer is applied to the component as a slurry including at least an organic solvent, the primary transition material and at least one slurry sintering aid, and where a reaction between the slurry sintering aid and the primary transition material results in the transition layer having a porosity of from 0% to about 15% by volume of the transition layer.

CROSS REFERENCE TO RELATED APPLICATIONS

This Application is a division application of U.S. patent applicationSer. No. 12/642,034, filed Dec. 18, 2009, which claims priority to U.S.Provisional Application Ser. No. 61/230,294, filed Jul. 31, 2009. Thedisclosures of these prior applications are incorporated herein byreference in their entirety.

TECHNICAL FIELD

Embodiments described herein generally relate to slurry compositions formaking solvent based environmental barrier coatings and environmentalbarrier coatings comprising the same.

BACKGROUND OF THE INVENTION

Higher operating temperatures for gas turbine engines are continuouslybeing sought in order to improve their efficiency. However, as operatingtemperatures increase, the high temperature durability of the componentsof the engine must correspondingly increase. Significant advances inhigh temperature capabilities have been achieved through the formulationof iron, nickel, and cobalt-based superalloys. While superalloys havefound wide use for components used throughout gas turbine engines, andespecially in the higher temperature sections, alternativelighter-weight component materials have been proposed.

Ceramic matrix composites (CMCs) are a class of materials that consistof a reinforcing material surrounded by a ceramic matrix phase. Suchmaterials, along with certain monolithic ceramics (i.e. ceramicmaterials without a reinforcing material), are currently being used forhigher temperature applications. These ceramic materials are lightweightcompared to superalloys yet can still provide strength and durability tothe component made therefrom. Therefore, such materials are currentlybeing considered for many gas turbine components used in highertemperature sections of gas turbine engines, such as airfoils (e.g.turbines, and vanes), combustors, shrouds and other like components thatwould benefit from the lighter-weight and higher temperature capabilitythese materials can offer.

CMC and monolithic ceramic components can be coated with environmentalbarrier coatings (EBCs) to protect them from the harsh environment ofhigh temperature engine sections. EBCs can provide a dense, hermeticseal against the corrosive gases in the hot combustion environment,which can rapidly oxidize silicon-containing CMCs and monolithicceramics. Additionally, silicon oxide is not stable in high temperaturesteam, but rather, can be converted to volatile (gaseous) siliconhydroxide species. Thus, EBCs can help prevent dimensional changes inthe ceramic component due to such oxidation and volatilizationprocesses. Unfortunately, there can be some undesirable issuesassociated with standard, industrial coating processes such as plasmaspray and vapor deposition (i.e. chemical vapor deposition, CVD, andelectron beam physical vapor deposition, EBPVD) currently used to applyEBCs.

Accordingly, there remains a need for methods for environmental barriercoatings for ceramic component that are suitable for use in the hightemperature steam environments present in gas turbine engines.

BRIEF DESCRIPTION OF THE INVENTION

Embodiments described herein generally relate to solvent based slurrycompositions for making environmental barrier coatings and toenvironmental barrier coatings formed therefrom, wherein the slurrycomposition comprises at least one slurry sintering aid.

Embodiments herein generally relate to environmental barrier coatingsfor high temperature ceramic components, the barrier coating comprising:a bond coat layer; an optional silica layer; and at least one transitionlayer including: from about 85% to about 100% by volume of thetransition layer of a primary transition material comprising a rareearth disilicate, or a doped rare earth disilicate; and from 0% to about15% by volume of the transition layer of a secondary material selectedfrom the group consisting of Fe₂O₃, iron silicates, rare earth ironoxides, Al₂O₃, mullite, rare earth aluminates, rare earthaluminosilicates, TiO₂, rare earth titanates, Ga₂O₃, rare earthgallates, NiO, nickel silicates, rare earth nickel oxides, Lnb metals,Lnb₂O₃, Lnb₂Si₂O₇, Lnb₂SiO₅, borosilicate glass, alkaline earthsilicates, alkaline earth rare earth oxides, alkaline earth rare earthsilicates, and mixtures thereof; wherein the transition layer is appliedto the component as a slurry comprising at least an organic solvent, theprimary transition material and at least one slurry sintering aid, andwherein a reaction between the slurry sintering aid and the primarytransition material results in the transition layer comprising aporosity of from 0% to about 15% by volume of the transition layer.

Embodiments herein also generally relate to environmental barriercoatings for high temperature ceramic components, the barrier coatingcomprising: a bond coat layer comprising silicon; an optional silicalayer; at least one transition layer including: from about 85% to about100% by volume of the transition layer of a primary transition materialcomprising a rare earth disilicate, or a doped rare earth disilicate;and from 0% to about 15% by volume of the transition layer of asecondary material; and any one or more of: an outer layer including:from about 85% to about 00% by volume of the outer layer of a primaryouter material comprising a rare earth monosilicate or a doped rareearth monosilicate, and from 0% to about 15% by volume of the outerlayer of the secondary material; an intermediate layer comprising theprimary outer material; and a compliant layer including: from about 85%to about 100% by volume of the compliant layer of a primary compliantmaterial selected from BSAS, and a rare earth doped BSAS; and from about0% to about 15% of a secondary compliant material selected from thegroup consisting of Ln₂O₃, Ln₂Si₂O₇, Ln₂SiO₅, Ln₃Al₅O₁₂, Al₂O₃, mullite,and combinations thereof wherein the transition layer, the outer layer,and the compliant layer are applied to the component as a slurry, atleast one slurry of the transition layer, outer layer, or compliantlayer comprises an organic solvent, the primary material and at leastone slurry sintering aid, and wherein a reaction between the slurrysintering aid and the primary material results in the transition layercomprising a porosity of from 0% to about 15% by volume of thetransition layer, the outer layer comprising a porosity of from about 0%to about 30% by volume of the outer layer, and the compliant layercomprising a porosity of from about 0% to about 15% by volume of thecompliant layer.

Embodiments herein also generally relate to environmental barriercoatings for high temperature ceramic components, the barrier coatingcomprising: a bond coat layer comprising silicon; an optional silicalayer; at least one transition layer including: from about 85% to about99% by volume of the transition layer of a primary transition materialcomprising a rare earth disilicate, or a doped rare earth disilicate;and from 1% to about 15% by volume of the transition layer of asecondary material; and any one or more of: an outer layer including:from about 85% to about 99% by volume of the outer layer of a primaryouter material comprising a rare earth monosilicate or a doped rareearth monosilicate, and from 1% to about 15% by volume of the outerlayer of the secondary material; an intermediate layer comprising theprimary outer material; and a compliant layer including: from about 85%to about 99% by volume of the compliant layer of a primary compliantmaterial selected from SAS, and a rare earth doped BSAS; and from about1% to about 15% of a secondary compliant material selected from thegroup consisting of Ln₂O₃, Ln₂Si₂O₇, Ln₂SiO₅, Ln₃Al₅O₁₂, Al₂O₃, mullite,and combinations thereof wherein the transition layer, the outer layer,and the compliant layer are applied to the component as a slurry, atleast one slurry of the transition layer, outer layer, or compliantlayer comprises an organic solvent, the primary material and at leastone slurry sintering aid, and wherein a reaction between the slurrysintering aid and the primary material results in the transition layercomprising a porosity of from 0.01% to about 15% by volume of thetransition layer, the outer layer comprising a porosity of from about0.01% to about 30% by volume of the outer layer, and the compliant layercomprising a porosity of from about 0.01% to about 15% by volume of thecompliant layer.

These and other features, aspects and advantages will become evident tothose skilled in the art from the following disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

While the specification concludes with claims particularly pointing outand distinctly claiming the invention, it is believed that theembodiments set forth herein will be better understood from thefollowing description in conjunction with the accompanying figures, inwhich like reference numerals identify like elements.

FIG. 1 is a schematic cross sectional view of one embodiment of acomponent having and environmental barrier coating in accordance withthe description herein;

FIG. 2 is a SEM cross-section of an EBC coating on a SiC—SiC CMC inaccordance with Example 1 herein;

FIG. 3 is a SEM cross-section of an EBC coating on SiC—SiC CMC inaccordance with Example 2 herein; and

FIG. 4 is a SEM cross-section of an EBC coating on SiC—SiC CMC inaccordance with Example 3 herein.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments described herein generally relate to solvent based slurrycompositions for making environmental barrier coatings and environmentalbarrier coatings comprising the same.

More specifically, the EBCs described herein comprise solvent basedslurries having sintering aids, which can lower the sinteringtemperature, thereby promoting the formation of dense EBC layers thatcan act as a hermetic seal to protect the underlying component fromcorrosion from the gases generated during high temperature combustionwithout damaging the component through exposure to high sinteringtemperatures, as explained herein below.

The EBCs described herein may be suitable for use in conjunction withCMCs or monolithic ceramics. As used herein, “CMCs” refers tosilicon-containing matrix and reinforcing materials. Some examples ofCMCs acceptable for use herein can include, but should not be limitedto, materials having a matrix and reinforcing fibers comprising siliconcarbide, silicon nitride, and mixtures thereof. As used herein,“monolithic ceramics” refers to materials comprising silicon carbide,silicon nitride, and mixtures thereof. Herein, CMCs and monolithicceramics are collectively referred to as “ceramics.”

As used herein, the term “barrier coating(s)” can refer to environmentalbarrier coatings (EBCs). The barrier coatings herein may be suitable foruse on “ceramic component,” or simply “component” 10 found in hightemperature environments (e.g. operating temperatures of above 2100° F.(1149° C.)), such as those present in gas turbine engines. Examples ofsuch ceramic components can include, for example, combustor components,turbine blades, shrouds, nozzles, heat shields, and vanes.

More specifically, EBC 12 may comprise a coating system includingvarious combinations of the following: a bond coat layer 14, an optionalsilica layer 15, at least one transition layer 16, an optional compliantlayer 18, an optional intermediate layer 22, and an optional outer layer20, as shown generally in FIG. 1 and as set forth herein below.

Bond coat layer 14 may comprise silicon metal, silicide, or acombination thereof, and may generally have a thickness of from about0.1 mils to about 6 mils (about 2.5 to about 150 micrometers). Due tothe application method as described herein below, there may be somelocal regions where the silicon bond coat is missing, which can beacceptable. For example, in one embodiment, bond coat layer can coverabout 100% of the surface of the component, and in another embodiment,about 90% or more of the surface area of the component.

As used herein “silicide” may include rare earth (Ln) silicides,chromium silicide (e.g. CrSi₃), niobium silicide (e.g. NbSi₂, NbSi₃),molybdenum silicide (e.g. MoSi₂, MoSi₃), tantalum silicide (e.g. TaSi₂,TaSi₃), titanium silicide (e.g. TiSi₂, TiSi₃), tungsten silicide (e.g.WSi₂, W₅Si₃), zirconium silicide (e.g. ZrSi₂), hafnium silicide (e.g.HfSi₂).

As used herein, “rare earth” represented “(Ln)” refers to the rare earthelements of scandium (Sc), yttrium (Y), lanthanum (La), cerium (Ce),praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm),europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium(Ho), erbium (Er), thulium (Tm), ytterbium (Yb), lutetium (Lu), andmixtures thereof.

Silica layer 15 can be amorphous or crystalline, and have an initialthickness of from about 0.0 mils to about 0.2 mils (about 0.0 to about 5micrometers). However, the thickness of silica layer 15 can increaseover time. Specifically, the silicon in bond coat layer 14 can oxidizeslowly during the service life of the EBC to gradually increase thethickness of silica layer 15. This oxidation of bond coat 14 can protectthe underlying ceramic component from oxidation since the bond coat isoxidized rather than the ceramic component. Silica layer 15 can, in someembodiments, also be doped with a doping composition, as defined hereinbelow, due to diffusion of the sintering aid into the silica layer.

Transition layer 16 may comprise a rare earth disilicate, a doped rareearth disilicate, or a doped rare earth disilicate containing secondarymaterials, as defined below. More specifically, transition layer 16 mayinclude from about 85% to about 100% by volume of the transition layerof a primary transition material and up to about 15% by volume of thetransition layer of a secondary material, and in one embodiment fromabout 85% to about 99% by volume of the transition layer of the primarytransition material and from about 1% to about 15% by volume of thetransition layer of the secondary material. In another embodiment,transition layer 16 may comprise 100% primary transition materialwherein the primary transition material can be doped, as describedbelow.

As used herein, “primary transition material” refers to a rare earthdisilicate (Ln₂Si₂O₇), or a doped rare earth disilicate. As used herein,“doped rare earth disilicate” refers to Ln₂Si₂O₇ doped with a “dopingcomposition” selected from the group consisting of iron (Fe), aluminum(Al), titanium (Ti), gallium (Ga), nickel (Ni), boron (B), an alkali, analkali-earth, and Lnb rare earths, as defined herein below. As usedherein throughout, “secondary material” refers to a material comprisinga doping composition (as defined previously), and specifically, can beselected from the group consisting of Fe₂O₃, iron silicates, rare earthiron oxides, Al₂O₃, mullite, rare earth aluminates, rare earthaluminosilicates, TiO₂, rare earth titanates, Ga₂O₃, rare earthgallates, NiO, nickel silicates, rare earth nickel oxides, Lnb metals,Lnb₂O₃, Lnb₂Si₂O₇, Lnb₂SiO₅, borosilicate glass, alkaline earthsilicates (for example, barium-strontium-aluminosilicates (BSAS)),alkaline earth rare earth oxides, alkaline earth rare earth silicates,and mixtures thereof. Any doping composition present in the primarymaterial should correspond to the doping composition contained in anysecondary material present (e.g. Fe-doped Ln₂Si₂O₇ with Fe₂O₃ secondarymaterial; Ti-doped Ln₂Si₂O₇ with TiO₂ secondary material; or Ni-dopedLn₂Si₂O₇ with rare earth nickel oxide secondary material, for example).

Each transition layer 16 may have a thickness of from about 0.1 mils toabout 40 mils (about 2.5 micrometers to about 1 millimeter), and may bemade and applied to the underlying layer as set forth below. In oneembodiment, there may be more than one transition layer present. In suchinstances, each transition layer may comprise the same or differentcombination of primary transition materials and secondary materials.Transition layer 16 may have a porosity level of from 0% to about 15% byvolume of the transition layer, and in another embodiment, from about0.01% to about 15% by volume of the transition layer.

Similarly, outer layer 20 may comprise a rare earth monosilicate, adoped rare earth monosilicate, or a doped rare earth monosilicatecontaining secondary material. More specifically, outer layer 20 caninclude from about 85% to about 100% by volume of the outer layer of aprimary outer material and up to about 15% by volume of the outer layerof the previously defined secondary material, and in one embodiment fromabout 85% to about 99% by volume of the outer layer of a primary outermaterial and from about 1% to about 15% by volume of the outer layer ofthe secondary material. In another embodiment, outer layer 20 maycomprise 100% primary outer material wherein the primary outer materialcan be doped as described below.

As used herein, “primary outer material” refers to a rare earthmonosilicate, or a doped rare earth monosilicate. As used herein, “dopedrare earth monosilicate” refers to Ln₂SiO₅ doped with a dopingcomposition, as defined previously. Outer layer 20 may have a thicknessof from about 0.1 mils to about 3 mils (about 2.5 to about 75micrometers), and may be made and applied to the underlying layer as setforth below. In one embodiment, outer layer 20 may have a porosity levelof from 0% to about 30% by volume of the outer layer, and in anotherembodiment, from about 0.01% to about 30% by volume of the outer layer,and in another embodiment, from about 0.01% to about 15% by volume ofthe outer layer. In some embodiments, outer layer 20 can comprise crackstherein at a density of up to about 10 cracks/mm that can form duringoperation due to thermal expansion anisotropy.

In reference to the embodiments herein, “Lnb rare earth metal”, orsimply “Lnb” refers to a sub-set of rare-earth metals having a meltingpoint below at least about 1450° C. including lanthanum, cerium,praseodymium, neodymium, promethium, samarium, europium, gadolinium,terbium, dysprosium, and ytterbium. In one embodiment, the sub-set caninclude only those rare earth elements having a melting point belowabout 1350° C. including lanthanum, cerium, praseodymium, neodymium,promethium, samarium, europium, gadolinium, and ytterbium. The Lnb rareearth metal can be utilized with SiC—SiC CMCs having an operation limitof about 1357° C.

As used herein throughout, “alkaline earth” can refer to magnesium (Mg),calcium (Ca), strontium (Sr), and barium (Ba). As used herein, “alkali”refers to lithium (Li), potassium (K), and sodium (Na). “Iron silicates”can include compounds such as Fe₂SiO₄, and glasses of rare earth ironsilicates. “Rare earth iron oxides” can include compounds such asgarnets (Ln₃Fe₅O₁₂), monoclinic ferrites (Ln₄Fe₂O₉), and perovskites(LnFeO₃). “Rare-earth aluminates” can include compounds such as garnets(Ln₃Al₅O₁₂), monoclinic aluminates (Ln₄Al₂O₉), and perovskites (LnAlO₃).“Rare earth aluminosilicates” can include glassy materials comprised ofabout 35-50 wt % Ln₂O₃, about 15-25 wt % Al₂O₃, and about 25-50 wt %SiO₂. “Rare-earth titanates” can include compounds such as Ln₂Ti₂O₇(pyrochlore) and Ln₂TiO₅. “Rare-earth gallates” can include compoundssuch as garnets (Ln₃Ga₅O₁₂), monoclinic gallates (Ln₄Ga₂O₉), perovskites(LnGaO₃), and Ln₃GaO₆. “Nickel silicates” can include compounds such asNi₂SiO₄. “Borosilicate glass” can refer to any amorphous materialcontaining up to about 15% by weight boron oxide (B₂O₃), up to about 10%alkali oxide selected from the group consisting of sodium (Na₂O),potassium (K₂O), lithium (Li₂O), or any combinations of thereof, up toabout 10% alumina (Al₂O₃), and a balance of silica (SiO₂). “Alkalineearth silicates” can include compounds such as Mg₂SiO₄, MgSiO₃, Ca₂SiO₄,Ca₃SiO₅, Ca₃Si₂O₇, CaSiO₃, Ba₂SiO₄, BaSiO₃, Ba₂Si₃O₈, BaSi₂O₅, Sr₂SiO₄,and SrSiO₃. “Alkali earth rare earth oxides” can include compounds suchas BaLn₂O₄, Mg₃Ln₂O₆, SrLn₂O₄, and Sr₂Ln₂O₅. “Alkaline earth rare earthsilicates” can include oxyapatite materials (i.e. Ae₂Ln₈Si₆O₂₆).

If present, compliant layer 18 may include from about 85% to about 100%by volume of the compliant layer of a primary compliant material and upto about 15% by volume of the compliant layer of a secondary compliantmaterial, and in one embodiment from about 85% to about 99% by volume ofthe compliant layer of a primary compliant material and from about 1% toabout 15% by volume of the compliant layer of the secondary compliantmaterial. In another embodiment, compliant layer 18 may comprise 100% byvolume of the compliant layer of a primary compliant material whereinthe primary compliant material may be doped with a rare earth element.

As used herein, “primary compliant material” refers to BSAS, or a rareearth doped BSAS, while “secondary compliant material” refers to Ln₂O₃,Ln₂Si₂O₇, Ln₂SiO₅, Ln₃Al₅O₁₂, Al₂O₃, mullite, and combinations thereof.Compliant layer 20 may have a thickness of from about 0.1 mils to about40 mils (about 2.5 micrometers to about 1 millimeter), and may be madeand applied as set forth below. In one embodiment, compliant layer 18may have a porosity level of from 0% to about 30% by volume of thecompliant layer, and in another embodiment, from about 0.01% to about30% by volume of the compliant layer, and in another embodiment, fromabout 0.01% to about 15% by volume of the compliant layer.

Intermediate layer 22, if present, can comprise the previously definedprimary outer materials of rare earth monosilicate or doped rare earthmonosilicate. Similar to the silica layer, intermediate layer 22 canform during the service life of the EBC. More specifically, hightemperature steam penetrates the outer layer 20, and as the steam reactswith the primary transition material of the transition layer tovolatilize SiO₂, intermediate layer 22 can form.

By way of example, and not limitation, the EBC systems described hereinmay include in one embodiment, component 10, bond coat layer 14, andtransition layer 16; in another embodiment, component 10, bond coatlayer 14, transition layer 16, and outer layer 20; in anotherembodiment, component 10, bond coat layer 14, transition layer 16,compliant layer 18, and outer layer 20; in another embodiment, component10, bond coat layer 14, transition layer 16, compliant layer 18,transition layer 16, and outer layer 20; in another embodiment,component 10, bond coat layer 14, silica layer 15, and transition layer16; in another embodiment, component 10, bond coat layer 14, silicalayer 15, transition layer 16, and outer layer 20; in anotherembodiment, component 10, bond coat layer 14, silica layer 15,transition layer 16, compliant layer 18, and outer layer 20; in anotherembodiment, component 10, bond coat layer 14, silica layer 15,transition layer 16, compliant layer 18, transition layer 16, and outerlayer 20; in another embodiment, component 10, bond coat layer 14,transition layer 16, intermediate layer 22, and outer layer 20; inanother embodiment, component 10, bond coat layer 14, silica layer 15,transition layer 16, intermediate layer 22, and outer layer 20; inanother embodiment, component 10, bond coat layer 14, silica layer 15,transition layer 16, intermediate layer 22 (which can form duringoperation), and outer layer 20; and in another embodiment, component 10,bond coat layer 14, silica layer 15, transition layer 16, compliantlayer 18, transition layer 16, intermediate layer 22 (which can formduring operation), and outer layer 20. Such embodiments can be suitablefor use in environments having a temperature up to about 1704° C. (3100°F.).

Alternately, the EBC system may comprise component 10, bond coat layer14, transition layer 16, and compliant layer 18; in another embodiment,component 10, bond coat layer 14, silica layer 15, transition layer 16,and compliant layer 18. Such embodiments can be suitable for use inenvironments having a temperature of up to about 1538° C. (2800° F.).

Those skilled in the art will understand that embodiments in addition tothose set forth previously are also acceptable, and that not all of thelayers need to be present initially, but rather, may form during engineoperation.

The EBC can be made and applied in accordance with the descriptionbelow.

Bond coat layer 14 may be applied by plasma spray processes, chemicalvapor deposition processes, electron beam physical vapor depositionprocesses, dipping in molten silicon, sputtering processes, and otherconventional application processes known to those skilled in the art.

As previously described, silica layer 15 can form during the servicelife of the EBC. Specifically, oxygen in the surrounding atmosphere candiffuse through any of the outer layer, compliant, and transitionlayer(s) present in the EBC and react with the silicon of bond coatlayer 14 to form silica layer 15. Alternately, silica layer 15 may beintentionally deposited by chemical vapor deposition, plasma spray,slurry deposition, or other conventional method.

Similar to silica layer 15, intermediate layer 22 can also form duringthe service life of the EBC when high temperature steam reacts withtransition layer 16, as previously described.

The manufacturing and application processes for transition layer 16,compliant layer 18 and outer layer 20 can consist of a solvent basedslurry deposition cycle including sintering aids to lower thetemperature needed to densify the layers. The slurry deposition cyclecan generally include slurry formation, slurry application, drying, andsintering, with optional masking, leveling, sintering aid infiltration,mask removal, and binder burnout steps, as set forth below. Thoseskilled in the art will understand that slurries of varying compositionscan be used to make EBC layers of varying composition and that multipleslurry deposition cycles can be used to build up the total thickness ofa particular layer. Each layer can have the thickness set forthpreviously with the average thickness per slurry deposition cycledepending primarily on the slurry solids loading, sintering aidconcentration, and number of dip, spray, or paint passes.

The slurries described in the embodiments herein can comprise variousslurry components, but generally include an organic solvent, ceramicparticles, sintering aid, and organic processing aids. Particularly, theslurry may comprise from about 6.8 wt % to about 96.1 wt % solvent, fromabout 0 wt % to about 8.9 wt % of a dispersant, from about 0 wt % toabout 15.4 wt % of a plasticizer, from about 3.9 wt % to about 93.2 wt %primary material, from about 0 wt % to about 15.4 wt % thickener, fromabout 0 wt % to about 1 wt % surfactant, and from about 0 wt % to about20 wt % slurry sintering aid if there is one sintering aid, oralternately, from about 0 wt % to about 79.9 wt % slurry sintering aidif there are two sintering aids present; and in another embodiment, fromabout 0.01 wt % to about 20 wt % slurry sintering aid if there is onesintering aid, or alternately, from about 0.01 wt % to about 79.9 wt %slurry sintering aid if there are two sintering aids present, asdescribed herein below.

More specifically, in such solvent based slurries, “organic solvent”refers to methanol, ethanol, propanol, butanol, pentanol, hexanol,heptanol, octanol, nonanol, decanol, dodecanol, acetone, methyl isobutylketone (MIBK), methyl ethyl ketone (MEK), toluene, ethylbenzene, propylbenzene, methoxybenzene, heptane, octane, nonane, decane, xylene,mineral spirits, naptha (such as VM & P naptha), tetrahydrofuran,ethers, and combinations thereof.

“Primary materials” may refer to Ln₂Si₂O₇, Ln₂SiO₅, or BSAS depending onwhich layer is being made.

“Dispersant” can refer to polyacrylic acid, polyacrylicacid-polyethylene oxide copolymers, polymethacrylic acid,polyethylenimine, phosphate esters, menhaden fish oil, polyethyleneoxide, polysilazane, and combinations thereof.

“Plasticizer” can refer to ethylene glycol, diethylene glycol,triethylene glycol, tetraethylene glycol glycerol, glycerin,polyethylene glycol, dibutyl phthalate, Bis(2-ethylhexyl) phthalate,Bis(n-butyl)phthalate, Butyl benzyl phthalate, Diisodecyl phthalate,Di-n-octyl phthalate, Diisooctyl phthalate, Diethyl phthalate,Diisobutyl phthalate, Di-n-hexyl phthalate, Di(propylene glycol)dibenzoate, Di(ethylene glycol) dibenzoate, tri(ethylene glycol)dibenzoate, and combinations thereof.

As used herein, “slurry sintering aid” can refer to sintering aidcompositions suitable for inclusion in the slurry. In some embodiments,there can be from about 0 wt % to about 20 wt %, and in some embodimentsfrom about 0.01 wt % to about 20 wt %, of a slurry sintering aidselected from iron oxide, gallium oxide, aluminum oxide, nickel oxide,titanium oxide, boron oxide, and alkaline earth oxides; carbonyl iron;iron metal, aluminum metal, boron, nickel metal, hydroxides includingiron hydroxide, gallium hydroxide, aluminum hydroxide, nickel hydroxide,titanium hydroxide, alkaline earth hydroxides; carbonates including ironcarbonate, gallium carbonate, aluminum carbonate, nickel carbonate,boron carbonate, and alkaline earth carbonates; oxalates including ironoxalate, gallium oxalate, aluminum oxalate, nickel oxalate, titaniumoxalate; and “solvent soluble salts” including solvent soluble ironsalts, solvent soluble gallium salts, solvent soluble aluminum salts,solvent soluble nickel salts, solvent soluble titanium salts, solventsoluble boron salts, and solvent soluble alkaline earth salts. In thecase of the compliant layer slurry, the “slurry sintering aid” mayinclude rare earth nitrate, rare earth acetate, rare earth chloride,rare earth oxide, ammonium phosphate, phosphoric acid, polyvinylphosphoric acid, and combination thereof.

In an alternate embodiment, the slurry can comprise from about 0 wt % toabout 59.3 wt %, and in one embodiment from about 0.01 wt % to about59.3 wt %, of an Lnb metal slurry sintering aid as defined previouslyherein, and from about 0 wt % to about 20.6 wt %, and in one embodimentfrom about 0.01 wt % to about 20.6 wt %, of a SiO₂ slurry sintering aid.In this embodiment, the Lnb and SiO₂ content can be held such that themole ratio of Lnb to SiO₂ is about 1 to 1 for slurries containing rareearth disilicate primary transition material, and about 2 to 1 forslurries containing rare earth monosilicate primary outer material.

As used herein, “solvent-soluble iron salts” can include ethoxide, iron2,4-pentanedionate, and iron tetramethylheptanedionate; “solvent-solublegallium salts” can include gallium 8-hydroxyquinolinate, gallium2,4-pentanedionate, gallium ethoxide, gallium isopropoxide, and gallium2,2,6,6-tetramethylheptanedionate; “solvent-soluble aluminum salts” caninclude butoxide, aluminum di-s-butoxide ethylacetoacetate, aluminumdiisopropoxide ethylacetoacetate, aluminum ethoxide, aluminumethoxyethoxyethoxide, aluminum 3,5-heptanedionate, aluminumisopropoxide, aluminum 9-octadecenylacetoacetate diisopropoxide,aluminum 2,4-pentanedionate, aluminum pentanedionatebis(ethylacetoacetate), aluminum 2,2,6,6-tetramethyl3,5-heptanedionate,and aluminum phenoxide; “solvent-soluble nickel salts” can includenickel 2,4-pentanedionate, nickel2,2,6,6-tetramethyl-3-5-heptanedionate; “solvent-soluble titanium salts”can include titanium allylacetoacetatetriisopropoxide, titaniumbis(triethanolamine)diisopropoxide, titanium butoxide, titaniumdi-n-butoxide bis(2-ethylhexanoate), titaniumdiisopropoxide(bis-2,4-pentanedionate), titanium diisopropoxidebis(tetramethylheptanedionate, titanium ethoxide, titaniumdiisopropoxide bis(ethylacetoacetate), titanium 2-ethylhexoxide,titanium iodide triisopropoxide, titanium isobutoxide, titaniumisopropoxide, titanium methacrylate triisopropoxide, titaniummethacryloxyethylacetoacetate triisopropoxide, titanium methoxide,titanium methoxypropoxide, titanium methylphenoxide, titaniumn-nonyloxide, titanium oxide bis(pentanedionate), titanium oxidebis(tetramethylheptanedionate), and titanium n-propoxide;“solvent-soluble boron salts” can include boron ethoxide, boronbutoxide, boron isopropoxide, boron methoxide, boron methoxyethoxide,boron n-propoxide; and “solvent-soluble alkaline earth salts” caninclude calcium isopropoxide, calcium methoxyethoxide, calciummethoxide, calcium ethoxide, strontium isopropoxide, strontiummethoxypropoxide, strontium 2,4-pentanedionate, strontium2,2,6,6-tetramethyl-3,5-heptanedionate, magnesium ethoxide, magnesiummethoxide, magnesium methoxyethoxide, magnesium 2,4-pentanedionate,magnesium n-propoxide, barium isopropoxide, barium methoxypropoxide,barium 2,4-pentanedionate, barium2,2,6,6-tetramethyl-3,5-heptanedionate.

“Thickener” refers to polyvinyl butyral, polyvinyl acetate,poly(isobutyl methacrylate), poly[(n-butyl methacrylate-co-isobutylmethacrylate)], methyl methacrylate copolymers, ethyl methacrylatecopolymers, poly methyl methacrylate, polyethyl methacrylate,polyvinylpyroline, ethyl cellulose, nitrocellulose, and other solventsoluble cellulose derivatives, and combinations thereof.

“Surfactant” refers to compositions selected from the group consistingof fluorocarbons, dimethylsilicones, and ethoxylated acetylenic diolchemistries (e.g. commercial surfactants in the Surfynol® series such asSurfynol® 420 and 502 (Air Products and Chemicals, Inc.)), andcombinations thereof.

Also, as used herein, “organic processing aids” refers to anydispersants, plasticizers, thickeners, and surfactants present in theslurry. These organic processing aids are comprised primarily of carbonand other elements that volatilize during processing such that they arenot present in the post-sintered coating.

The slurry can be formed by combining any or all of the previouslydescribed slurry components with mixing media in a container. Themixture can be mixed using conventional techniques known to thoseskilled in the art such as shaking with up to about a 1 inch (about 25.4mm) diameter alumina or zirconia mixing media, ball milling using abouta 0.25 inch to about a 1 inch (about 0.64 cm to about 2.54 cm) diameteralumina or zirconia mixing media, attritor milling using about a 1 mm toabout a 5 mm diameter zirconia-based mixing media, planetary ballmilling using from about a 1 mm to about a 5 mm diameter zirconia-basedmedia, or mechanical mixing or stirring with simultaneous application ofultrasonic energy. The mixing media or ultrasonic energy can break apartany agglomerated ceramic particles in the slurry. Any mixing mediapresent may then be removed by straining, for example.

If not added previously, thickener may be added to the slurry if desiredand the resulting mixture may be agitated by such methods as mechanicalstirring, rolling, blending, shaking, and other like methods until thethickener is fully dissolved, generally after about 5 to about 60minutes.

Similarly, if not added previously, the addition of the sintering aidsmay follow along with mixing using the previously described methodsuntil the sintering aids dissolve, which is about 5 to about 60 minutes.

Once all slurry components have been mixed, the slurry can be filteredthrough screens of varying mesh sizes to remove any impurities that maybe present, such as after the initial mixing of the slurry or after useof the slurry to deposit coating layers. A 325 mesh screen, for example,can be used to filter out impurities having an average size of about 44microns or greater.

After mixing and optional filtering, the slurry can be agitatedindefinitely by slow rolling, slow mechanical mixing, or other likemethods to avoid trapping air bubbles in the slurry. In one embodiment,the slurry may be refreshed by adding additional solvent to account forthat which has evaporated during processing. Alternately, once mixed,the slurry can be set aside until needed for application. Those skilledin the art will understand that the previous embodiment sets forth onemethod for making the slurry compositions described herein, and thatother methods are also acceptable, as set forth in the Examples below.

Optionally, masking can be applied to the ceramic component before theslurry is applied to prevent coating specific areas of the component.Masking may be carried out using conventional techniques known to thoseskilled in the art including, but not limited to, tapes, tooling, andpaint-on adhesives.

Once all desired masking of the ceramic component is complete, theslurry can be applied to the component to produce a coated component.The slurry can be applied to the component (or on top of a previouslyapplied layer) using any conventional slurry deposition method known tothose skilled in the art, including but not limited to, dipping thecomponent into a slurry bath, or painting, rolling, stamping, spraying,or pouring the slurry onto the component. Slurry application can becarried out manually or it may be automated.

Once the slurry has been applied to the component, and while the slurryis still wet, it may be leveled to remove excess slurry material.Leveling may be carried out using conventional techniques such as, butnot limited to, spinning, rotating, slinging the component, drippingwith or without applied vibration, or using a doctor blade, to removeexcess slurry material. Similar to the slurry application, leveling canbe conducted manually or it may be automated.

Next, the coated component can be dried. Drying may be carried out in anenclosed area having additional organic solvent present in secondarycontainers. This can help slow the drying process because the atmospherecan be saturated with organic solvent. Since this process utilizes anorganic solvent, it is not strongly sensitive to humidity. Temperaturevariation can be used to control the drying rate, however, those skilledin the art will understand that that temperatures can be kept below theflash point of the organic solvent. Placing a coated component in avacuum chamber and pulling a vacuum can also be used to acceleratedrying.

After drying, any masking present may then be removed by peeling offtapes and adhesives, pyrolysis of tapes and adhesives, or removingmulti-use tooling. Any rough edges remaining after masking removal maybe scraped or cut away using a sharp or abrasive tool.

Next, burnout of the organic processing aids may be carried out byplacing the dried component in an elevated temperature environment sothat any residual solvent can be evaporated and the organic processingaids can be pyrolyzed. In one embodiment, burnout of the organicprocessing aids may be accomplished by heating the dried component at arate of from about 1° C./min to about 15° C./min to a temperature offrom about 400° C. to about 1000° C. and holding the component at thistemperature for from about 0 to about 10 hours. In another embodiment,the coated component may be heated at a rate of from about 2° C./min toabout 6° C./min to a temperature of from about 600° C. to about 800° C.and holding the component at this temperature for from about 0 to about10 hours. In another embodiment, the hold time can be eliminated byslowly ramping up to the target temperature without holding, followed byramping up or down to another temperature at a different rate. Inanother embodiment, binder burnout can occur rapidly by placing thecoated component into a furnace heated to a temperature of from about400° C. to about 1400° C.

Binder burnout and sintering heat treatments may be carried out in anambient air atmosphere, or in an inert gas atmosphere where the inertgas is selected from hydrogen, a noble gas such as helium, neon, argon,krypton, xenon, or mixtures thereof. In one embodiment, the inert gasatmosphere can be used in conjunction with Lnb metal and SiO₂ sinteringaids so as not to convert the rare earth metal to an oxide before itmelts. Maintaining the Lnb metal in a metal state can promote liquidphase sintering and subsequent reaction with the SiO₂.

The dried component may then be sintered to produce a componentcomprising an environmental barrier coating. Sintering can serve tosimultaneously densify and impart strength to the coating. Additionally,in the case of the transition and compliant layers of the EBC, sinteringcan impart a hermetic seal against high temperature steam present in theengine environment. In the case of the outer layer, sintering canprovide a dense barrier against the infiltration of molten silicates,such as calcium-magnesium aluminosilicate (CMAS) that may be encounteredas a result of particulate contamination (i.e. dirt and sand) in theengine environment. Sintering can be carried out using a conventionalfurnace, or by using such methods as microwave sintering, lasersintering, infrared sintering, and the like.

Sintering can be accomplished by heating the dried component at a rateof from about 1° C./min to about 15° C./min to a temperature of fromabout 1100° C. to about 1700° C. and holding the component at thattemperature for from about 0 to about 24 hours. In another embodiment,sintering can be accomplished by heating the coated component at a rateof from about 5° C./min to about 15° C./min to a temperature of fromabout 1300° C. to about 1375° C. and holding the component at thattemperature for from about 0 to about 24 hours. In another embodiment,sintering can occur rapidly by placing the coated component into afurnace heated to a temperature of from about 1000° C. to about 1400° C.

In an alternate embodiment, all layers of the EBC can be applied, one ontop of the other, before masking removal, organic processing aidburnout, and sintering are carried out. Those skilled in the art willunderstand that after application of each layer, the layer should bedried, or partially dried, before the application of the subsequentlayer.

In another embodiment, the sintering aid does not need to be addeddirectly to the transition or outer layer of the slurry to achieve thedesired result. The sintering aid can be added to one layer of the EBCslurry, and during sintering, the sintering aid can diffuse throughoutthe EBC to the remaining layers. In another embodiment, a primarymaterial slurry with no sintering aid can be densified by applying thelayer, allowing it to dry, and then back infiltrating a sol-gel solutioncomprising a sintering aid prior to heat treatment as explained below.

Infiltration may allow for the densification of a thicker layer of EBCmaterial at one time. Moreover, infiltration is a way to add moresintering aid after sintering if the coating is not as dense as desired.The sol-gel solution used for infiltration may be a solution of anorganic solvent and a solvent soluble salt sintering aid, as definedpreviously, or a solution of water and a water soluble salt sinteringaid.

As used herein “water soluble salt sintering aid” can refer to“water-soluble iron salts” such as iron nitrate and iron acetate;“water-soluble gallium salts” such as gallium nitrate and galliumacetate; “water-soluble aluminum salts” such as aluminum nitrate andaluminum acetate; “water-soluble nickel salts” such as nickel nitrateand nickel acetate; “water-soluble titanium salts” such as titaniumchloride; “water-soluble boron salts” such as boric acid and ammoniumborate; and “water-soluble alkaline earth salts” such as includeMg(NO₃)₂, Ca(NO₃)₂, Sr(NO₃)₂, Ba(NO₃)₂, MgC₂H₃O₂, CaC₂H₃O₂, SrC₂H₃O₂,and BaC₂H₃O₂.

As used herein, “sintering aid(s)” can refer to any of a “slurrysintering aid,” a “solvent soluble sintering aid,” or a “water solublesalt sintering aid,” as defined previously. Without intending to belimited by theory, the inclusion of sintering aids to the EBCembodiments herein can increase the rate of diffusion of primarymaterial such that surface area reduction (i.e. high surface areaparticles consolidating to form a dense coating) can occur at lowertemperatures than it would have absent the sintering aid. As previouslydescribed, sintering at lower temperatures (i.e. about 1357° C. orbelow) can not only result in a highly dense (i.e. greater than about85% for the transition layer, greater than about 70% for the compliantlayer, and greater than about 70% for the outer layer) coating that canbe less susceptible to the penetration of hot steam from the engineenvironment, but can also help prevent the degradation of the mechanicalproperties of the underlying component that could result from prolongedexposure to higher temperatures.

Sintering aids can act in a variety of ways depending on the amount ofsintering aid included in the EBC and the time at which the coating isexposed to sintering temperatures. For example, in one embodiment, thesintering aid can dissolve completely into the primary material (i.e.primary transition, outer, or compliant, material) to “dope” thematerial. In another embodiment, if the amount of sintering aid that issoluble in the primary material is exceeded, the remaining insolubleportion of sintering aid can react with the primary material to form thesecondary material (i.e. secondary transition, outer, or compliantmaterial). In another embodiment, primary material and secondarymaterial can be present as described previously, along with residualsintering aid.

In these latter two embodiments, when the secondary material is highlyvolatile in high temperature steam, such as but not limited to, alkalisilicates, alkaline earth silicates, mullite, iron silicate,borosilicate glass, nickel silicate, and residual sintering aids ofiron, aluminum, titanium, gallium, nickel, boron, alkali, andalkali-earth compounds, as long as the total volume of secondarymaterial, plus porosity (plus residual sintering aid when present) ineither of the intermediate layer or compliant layer (when present) ofthe EBC remains about 15% by volume or less, the hermetic seal can bemaintained. Alternately, in these latter two embodiments, when thesecondary material is highly resistant to volatilization in hightemperature steam, such as when the secondary material comprises a rareearth containing compound, such as but not limited to rare earth oxide,rare earth titanate, rare earth iron compound, rare earth gallate, rareearth aluminate, and rare earth aluminosilicate, the porosity in eitherof the intermediate or compliant layer (when present) of the EBC needremain about 15% by volume or less to maintain the hermetic seal.

It should be noted that at low levels of sintering aid, the densifiedcoating layer might not initially include any detectable secondarymaterials. In some embodiments, the secondary materials may never becomedetectable. In other embodiments, however, after hours of exposure tohigh temperature steam in the engine environment, the secondarymaterials can become detectable using techniques such as x-raydiffraction, electron microscopy, electron dispersive spectroscopy(EDS), and the like.

EBC embodiments described herein can offer a variety of benefits overcurrent EBCs and manufacturing processes thereof. Specifically, aspreviously described, the inclusion of a sintering aid in the EBCembodiments herein can permit sintering at lower temperatures (i.e.about 1357° C. or below). This can result in a highly dense (i.e.greater than about 85% for the transition layer, and greater than about70% for each of the outer, and compliant, layers) coating that can beless susceptible to the penetration of hot steam from the engineenvironment, and can also help prevent the degradation of the mechanicalproperties of the underlying component that could result from prolongedexposure to higher temperatures. Also, the embodiments set forth hereincan be made at less expense than current EBCs due to the use of theslurry deposition process, which is made possible by the incorporationof sintering aids into the various layers. Moreover, the presentembodiments can provide for EBCs having a more uniform thickness thanconventional techniques, such as plasma spraying, even when applyingthin layers (<2 mils, or less than about 50 micrometers). Additionally,the slurry deposition process can allow for the application of the EBCsto internal component passages as well as the ability to produce smoothsurface finishes without an additional polishing step.

There can be occasions when the EBC develops small and/or narrow defects(e.g. about 10 microns to about 5 mm in diameter; or about 10 microns toabout 1 mm in width) that need to be repaired. The following repairprocesses are applicable to the EBCs described herein and may be carriedout after sintering of an individual EBC layer, or after sintering theentire applied EBC, as explained herein below.

In one embodiment, repairs may include remedying defects in one or moreindividual layers as the EBC is being applied using the methodsdescribed herein. In this embodiment, the repair can be carried outafter sintering a given layer by applying a repair slurry comprising thesame slurry materials used to make the layer having the defects. Forexample, if the transition layer develops a defect after sintering, thedefect could be repaired using a “transition layer repair slurry” thatcomprises the same transition layer slurry materials used in theoriginal application of the transition layer. In one embodiment, therepair slurry can comprise a higher solids loading of primary materialceramic particles than the original slurry layer as this can reduceshrinkage on drying and sintering of the repaired portion of thecoating. In particular, the solids loading of primary material ceramicparticles in the repair slurry can be greater than about 30% to about55% by volume (as opposed to greater than about 10% by volume in oneembodiment of the original slurry, and from about 10% to about 55% byvolume in another embodiment of the original slurry used to make thelayer). The repair slurry may be applied using any conventional methodincluding those described previously, and the resulting “repair(ed)coating” may then be processed as described previously herein beforeapplication of any subsequent layer of the EBC.

In an alternate embodiment, repairs may include fixing defects afterapplication and sintering of the entire EBC. In this embodiment, therepair may be carried out on the EBC having defects using a transitionlayer repair slurry comprising the same materials present in thepreviously defined transition layer slurry (i.e. primary transitionmaterial, a sintering aid, and optionally secondary material). Thisparticular repair slurry can seep into any defects present in the EBCand provide a hermetic seal to the repaired EBC coating after sintering.Again, the solids loading of the transition layer repair slurry maycomprise upwards of about 30% to 55% by volume.

Additionally, repair processes may be used to reduce surface roughnessof a plasma sprayed EBC having any composition. Specifically, if thesurface roughness of a plasma sprayed EBC is unacceptable the coatingcan be smoothed over by applying either of the previously describedtransition layer slurry or outer layer slurry. When applied over theplasma sprayed EBC, the transition layer slurry or outer layer slurrycan fill in any gaps, grooves, or uneven portions of the plasma sprayedcoating and reduce the surface roughness to an acceptable degree. Morespecifically, depending on the thickness of the transition layer slurryor outer layer slurry, surface roughness of the plasma sprayed EBC canbe reduced from greater than 200 micro inch (about 5 micrometers) Ra, tobetween 40 micro inch Ra and 200 micro inch (about 1 to about 5micrometers) Ra in one embodiment, and from between 40 micro inch Ra to150 micro inch (about 1 to about 3.8 micrometers) Ra in anotherembodiment. In one embodiment, the transition layer slurry or outerlayer slurry can comprise a thickness of at least about 0.5 mils (about12.5 micrometers), and in another embodiment from about 0.5 mils toabout 3 mils (about 12.5 to about 75 micrometers). The appliedtransition layer slurry can then be processed as described previously toproduce a repaired EBC having an acceptable surface roughness.Additional slurry layers may be applied to the EBC if desired.

Such repair processes can provide the ability to repair localizeddefects, at varying points during the application or life of thecoating, as opposed to stripping off and reapplying the entire coating.This, in turn, can result in a savings of time, labor, and materials.

EXAMPLES Example 1

A silicon bond coat was applied to a SiC—SiC CMC using a conventionalair plasma spray process. Next, a primary transition material slurry wasmade by first mixing ytterbium disilicate (primary transition material),iron oxide nanoparticles (sintering aid), ethanol (solvent), andpolyethylenimine (dispersant) in a plastic container, along with enough0.25 inch (6.35 mm) diameter, spherical zirconia media to line thebottom of container. This mixture was placed on a roller mill for 15hours. After taking the container off of the roller mill, the zirconiamedia was removed and the slurry was filtered through a 325 mesh screento remove any large particle agglomerates.

The resulting primary transition material slurry (Slurry A) consisted of56.11% ytterbium disilicate, 0.54% iron oxide, 0.57% polyethylenimine,and the balance ethanol (all percents by weight). The silicon-coatedceramic component was dipped into Slurry A, dried in ambient conditions,re-dipped into Slurry A, dried in ambient conditions, and heat-treatedat 10° C./minute to 1000° C. to burn out any residual organic materials.Then, the temperature was increased at 10° C./minute from 1000° C. to1344° C. and held for 10 hours to form a transition layer comprisingiron-doped ytterbium disilicate. The entire heat treatment was carriedout in air. The heating environment resulted in the transition layerhaving a porosity of less than 15% by volume. These dipping, drying, andheat treating processes were repeated 8 times to build thickness of thetransition layer.

Next, a primary outer material slurry was made by first mixing ytterbiummonosilicate (primary outer material), iron oxide nanoparticles(sintering aid), ethanol (solvent), and polyethylenimine (dispersant) ina plastic container, along with enough 0.25 inch (6.35 mm) diameter,spherical zirconia media to line the bottom of container. This mixturewas placed on a roller mill for 15 hours. After taking the container offof the roller mill, the zirconia media was removed and the slurry wasfiltered through a 325 mesh screen to remove any large particleagglomerates.

The resulting primary outer material slurry (Slurry B) consisted of61.19% ytterbium monosilicate, 0.29% iron oxide, 0.61% polyethylenimine,and the balance ethanol (all percents by weight). The silicon andtransition layer-coated ceramic component was dipped into Slurry B,dried in ambient conditions, re-dipped into Slurry B, dried in ambientconditions, and heat-treated at 10° C./minute to 1000° C. to burn outany residual organic materials. Then, the temperature was increased at10° C./minute from 1000° C. to 1344° C. and held for 10 hours to densifythe transition layer. After heat treatment, the layer comprisediron-doped yttrium monosilicate. The entire heat treatment was carriedout in air. The heating environment resulted in the transition layerhaving a porosity of less than 15% by volume. The dipping, drying, andheat treatments were repeated 2 times to build thickness of the outerlayer.

FIG. 2 shows a SEM micrograph of this coating microstructure with theair plasma spray silicon bond coat (100), a thin silica layer (3.9micrometers) (102), transition layer (104), and outer layer (106). Theouter layer and transition layer appeared to consist only of iron-dopedprimary materials (iron-doped ytterbium monosilicate and iron-dopedytterbium disilicate, respectively), x-ray diffraction was not able todetect any secondary material present in the EBC. EDS analysis of thelayers suggested that the iron oxide had dissolved into the primarymaterials.

Example 2

A CMC (101) coated with the EBC of Example 1 was exposed to 2400° F.(1316° C.) steam for 1000 hours. FIG. 3 shows a SEM micrograph of thiscoating after high temperature steam exposure with silicon bond coat(100), a silica layer (102) that has grown to approximately 7.0micrometer thickness, an iron-doped ytterbium disilicate transitionlayer (104), and a dense ytterbium monosilicate outer layer (106) withcracks. Some ytterbium disilicate has formed around the cracks in themonosilicate layer as an artifact of high gaseous silicon content in thestatic atmosphere of the steam test.

Example 3

To demonstrate proof of principle, a primary transition layer wasdeposited on a silicon metal wafer using a slurry deposition process. Aprimary transition material slurry was made by first mixing ytterbiumdisilicate powder (primary transition material), gallium oxide powder(sintering aid), ethanol (solvent), and polyethylenimine (dispersant) ina plastic container, along with enough 0.25 inch (6.35 mm) diameter,spherical zirconia media to line the bottom of container. This mixturewas placed on a roller mill for 15 hours. After taking the container offof the roller mill, the zirconia media was removed and the slurry wasfiltered through a 325 mesh screen to remove any large particleagglomerates.

The resulting primary transition material slurry (Slurry C) consisted of56.35% ytterbium disilicate, 0.64% gallium oxide, 0.57%polyethylenimine, and the balance ethanol (all percents by weight). Thesilicon-coated ceramic component was dipped into Slurry C, dried inambient conditions, re-dipped into Slurry C, dried in ambientconditions, and heat-treated at 10° C./minute to 1000° C. to burn outany residual organic materials. Then, the temperature was increased at10° C./minute from 1000° C. to 1344° C. and held for 10 hours to form atransition layer comprising iron-doped ytterbium disilicate. The entireheat treatment was carried out in air. The heating environment resultedin the transition layer (108) having a porosity of less than 15% byvolume as shown in FIG. 4. No secondary phases were observed in thiscoating via electron microscope examination. EDS analysis suggested thatthe gallium had dissolved into the ytterbium disilicate primarymaterial.

This written description uses examples to disclose the invention,including the best mode, and also to enable any person skilled in theart to make and use the invention. The patentable scope of the inventionis defined by the claims, and may include other examples that occur tothose skilled in the art. Such other examples are intended to be withinthe scope of the claims if they have structural elements that do notdiffer from the literal language of the claims, or if they includeequivalent structural elements with insubstantial differences from theliteral language of the claims.

What is claimed is:
 1. An environmental barrier coating for hightemperature ceramic components, the barrier coating comprising: a bondcoat layer; and at least one transition layer including: from about 85%to about 100% by volume of a primary transition material comprising adoped rare earth disilicate containing a doping composition selectedfrom the group consisting of iron, aluminum, titanium, gallium, nickel,boron, an alkali, and alkali earth, the doped rare earth disilicatebeing a rare earth disilicate in which is dissolved at least one slurrysintering aid containing the doping composition; and up to about 15% byvolume of a secondary material containing the doping composition of theprimary transition material, the secondary material being selected fromthe group consisting of Fe2O3, iron silicates, rare earth iron oxides,Al2O3, mullite, rare earth aluminates, rare earth aluminosilicates,TiO2, rare earth titanates, Ga2O3, rare earth gallates, NiO, nickelsilicates, rare earth nickel oxides, borosilicate glass, alkaline earthsilicates, alkaline earth rare earth oxides, alkaline earth rare earthsilicates, and mixtures thereof; wherein the transition layer is appliedto the component as a slurry comprising at least an organic solvent, theprimary transition material and the slurry sintering aid; and thetransition layer has a porosity of from 0% to about 15% by volume of thetransition layer an outer layer including: from about 85% to about 100%by volume of a primary outer material comprising a rare earthmonosilicate or a doped rare earth monosilicate, and from 0% to about15% by volume of the secondary material; and a compliant layerincluding: from about 85% to about 99% by volume of a primary compliantmaterial selected from BSAS and a rare earth doped BSAS, and from about1% to about 15% of a secondary compliant material selected from thegroup consisting of Ln₂O₃, Ln₂Si₂O₇, Ln₂SiO₅, Ln₃Al₅O₁₂, Al₂O₃, mullite,and combinations thereof.
 2. The coating of claim 1 wherein the solventis selected from the group consisting of methanol, ethanol, propanol,butanol, pentanol, hexanol, heptanol, octanol, nonanol, decanol,dodecanol, acetone, methyl isobutyl ketone, methyl ethyl ketone,toluene, ethylbenzene, propyl benzene, methoxybenzene, heptane, octane,nonane, decane, xylene, mineral spirits, naptha, tetrahydrofuran,ethers, and mixtures thereof.
 3. The coating of claim 1 comprising morethan one transition layer, wherein each transition layer comprises adifferent combination of primary transition materials and secondarytransition materials.
 4. The coating of claim 1 wherein the outer layercomprises a porosity of from about 0% to about 30% by volume of theouter layer, and the compliant layer comprises a porosity of from about0% to about 15% by volume of the compliant layer.
 5. The coating ofclaim 4 wherein the coating comprises the outer layer, the primary outermaterial of the outer layer is the doped rare earth monosilicate, andthe doped rare earth monosilicate contains an outer layer dopingcomposition selected from the group consisting of iron, aluminum,titanium, gallium, nickel, boron, an alkali, and alkali earth, and anLnb rare earth metal.
 6. The coating of claim 4 wherein the transitionlayer comprises a thickness of from about 2.5 micrometers to about 1millimeter, the outer layer comprises a thickness of from about 2.5 toabout 75 micrometers, and the compliant layer comprises a thickness offrom about 2.5 micrometers to about 1 millimeter.
 7. The coating ofclaim 4 comprising an intermediate layer including the primary outermaterial.
 8. The coating of claim 4 wherein the outer layer includesfrom about 85% to about 99% by volume of the primary outer material,from about 1% to about 15% by volume of the secondary material.
 9. Thecoating of claim 1 wherein the high temperature ceramic componentcomprises a ceramic matrix composite or a monolithic ceramic turbineengine component selected from the group consisting of combustorcomponents, turbine blades, shrouds, nozzles, heat shields, and vanes.10. The coating of claim 1 wherein the transition layer contains aresidual amount of the slurry sintering aid, and the slurry sinteringaid is selected from the group consisting of iron oxide, gallium oxide,aluminum oxide, nickel oxide, titanium oxide, boron oxide, alkalineearth oxides, carbonyl iron, iron metal, aluminum metal, boron, nickelmetal, iron hydroxide, gallium hydroxide, aluminum hydroxide, nickelhydroxide, titanium hydroxide, alkaline earth hydroxides, ironcarbonate, gallium carbonate, aluminum carbonate, nickel carbonate,boron carbonate, alkaline earth carbonates, iron oxalate, galliumoxalate, aluminum oxalate, nickel oxalate, titanium oxalate, solventsoluble iron salts, solvent soluble gallium salts, solvent solublealuminum salts, solvent soluble nickel salts, solvent titanium salts,solvent soluble boron salts, and solvent soluble alkaline earth salts.11. An environmental barrier coating for high temperature ceramiccomponents, the barrier coating comprising: a bond coat layer comprisingsilicon; a silica layer; at least one transition layer including: fromabout 85% to about 99% by volume of a primary transition materialcomprising a doped rare earth disilicate containing a doping compositionselected from the group consisting of iron, aluminum, titanium, gallium,nickel, boron, an alkali, and alkali earth the doped rare earthdisilicate being a rare earth disilicate in which is dissolved at leastone slurry sintering aid containing the doping composition; and from 1%to about 15% by volume of a secondary material containing the dopingcomposition of the primary transition material, the secondary materialbeing selected from the group consisting of Fe2O3, iron silicates, rareearth iron oxides, Al2O3, mullite, rare earth aluminates, rare earthaluminosilicates, TiO2, rare earth titanates, GaO3, rare earth gallates,NiO, nickel silicates, rare earth nickel oxides, borosilicate glass,alkaline earth silicates, alkaline earth rare earth oxides, alkalineearth rare earth silicates, and mixtures thereof; and an outer layerincluding: from about 85% to about 99% by volume of a primary outermaterial comprising a rare earth monosilicate or a doped rare earthmonosilicate, and from 1% to about 15% by volume of the secondarymaterial; an intermediate layer comprising the primary outer material;and a compliant layer including: from about 85% to about 99% by volumeof a primary compliant material selected from BSAS, and a rare earthdoped BSAS, and from about 1% to about 15% of a secondary compliantmaterial selected from the group consisting of Ln₂O₃, Ln₂Si₂O₇, Ln₂SiO₅,Ln₃Al₅O₁₂, Al₂O₃, mullite, and combinations thereof; wherein thetransition layer, the outer layer, and the compliant layer are appliedto the component as a slurry, the slurry for each of the transitionlayer, the outer layer, and the compliant layer comprises an organicsolvent, the slurry sintering aid, and a corresponding one of theprimary transition material, the primary outer material, or the primarycompliant material; wherein the transition layer has a porosity of from0.01% to about 15% by volume of the transition layer, the outer layerhas a porosity of from about 0.01% to about 30% by volume of the outerlayer, and the compliant layer has a porosity of from about 0.01% toabout 15% by volume of the compliant layer.
 12. The coating of claim 11wherein the solvent is selected from the group consisting of methanol,ethanol, propanol, butanol, pentanol, hexanol, heptanol, octanol,nonanol, decanol, dodecanol, acetone, methyl isobutyl ketone, methylethyl ketone, toluene, ethylbenzene, propyl benzene, methoxybenzene,heptane, octane, nonane, decane, xylene, mineral spirits, naptha,tetrahydrofuran, ethers, and mixtures thereof.
 13. The coating of claim11 wherein the primary outer material of the outer layer is the dopedrare earth monosilicate, and the doped rare earth monosilicate containsan outer layer doping composition selected from the group consisting ofiron, aluminum, titanium, gallium, nickel, boron, an alkali, and alkaliearth, and an Lnb rare earth metal.
 14. The coating of claim 11 whereinthe transition layer comprises a thickness of from about 2.5 micrometersto about 1 millimeter, the outer layer comprises a thickness of fromabout 2.5 to about 75 micrometers, and the compliant layer comprises athickness of from about 2.5 micrometers to about 1 millimeter.
 15. Thecoating of claim 11 comprising an intermediate layer including theprimary outer material, wherein the high temperature ceramic componentcomprises a ceramic matrix composite or a monolithic ceramic turbineengine component selected from the group consisting of combustorcomponents, turbine blades, shrouds, nozzles, heat shields, and vanes.16. The coating of claim 15 wherein: each of the transition layer, theouter layer, and the compliant layer contains a residual amount of theslurry sintering aid thereof; the slurry sintering aid used with thetransition layer slurry and the outer layer slurry is selected from thegroup consisting of iron oxide, gallium oxide, aluminum oxide, nickeloxide, titanium oxide, boron oxide, alkaline earth oxides, carbonyliron, iron metal, aluminum metal, boron, nickel metal, iron hydroxide,gallium hydroxide, aluminum hydroxide, nickel hydroxide, titaniumhydroxide, alkaline earth hydroxides, iron carbonate, gallium carbonate,aluminum carbonate, nickel carbonate, boron carbonate, alkaline earthcarbonates, iron oxalate, gallium oxalate, aluminum oxalate, nickeloxalate, titanium oxalate, solvent soluble iron salts, solvent solublegallium salts, solvent soluble aluminum salts, solvent soluble nickelsalts, solvent titanium salts, solvent soluble boron salts, and solventsoluble alkaline earth salts; and the slurry sintering aid used with thecompliant layer slurry is selected from the group consisting of rareearth nitrate, rare earth acetate, rare earth chloride, rare earthoxide, ammonium phosphate, phosphoric acid, polyvinyl phosphoric acid,and combination thereof.